1. Field of the Invention
The present invention relates to protected 2′-deoxycytidines and hydrated crystals thereof and, particularly, to a method for producing and purifying 5′ position-protected 2′-deoxycytidines.
The 5′ position-protected 2′-deoxycytidines are useful compounds as raw materials for antisense DNA, which have been developed in recent years.
2. Description of the Related Art
Recently, antisense DNA drugs have been intensively developed with progress in the development of genomic medicines. The progress has raised demands for DNA oligomers, the raw materials for antisense DNA drugs, and protected deoxynucleosides, the raw materials for DNA oligomers. On the other hand, applications for producing medicines require the use of extremely high-purity intermediate products to minimize the generation of byproducts attributed to impurities.
Japanese Unexamined Patent Application Publication Nos. 58-180500 and 63-179889 and PCT Japanese Translation Patent Publication No. 06-507883 disclose that 5′ position-protected deoxynucleosides can be purified by column chromatography. This method can relatively easily remove impurities having largely different polarities and structures, but cannot satisfactorily remove impurities having similar structures. For example, it is particularly difficult to remove impurities having 5′- and 3′-hydroxyl groups protected by trityl derivatives such as dimethoxytrityl groups, and those having an unrequired 3′-hydroxyl group, instead of a required 5′-hydroxyl group, protected by a trityl derivative such as a dimethoxytrityl group. In addition, a mixture of these impurities significantly affects the subsequent steps in the production of medicines. Furthermore, column chromatography involves a large-scale purifying apparatus and a large amount of solvent in industrial production; hence, this method will be unsuitable for mass production of large quantities of product in the future.
Methods other than column chromatography for removing impurities have been studied. Specifically, purification by reprecipitation is disclosed in Japanese Unexamined Patent Application Publication No. 60-152495 and PCT Publication Nos. WO00/75154 and WO00/39138. In the reprecipitation method, a poor solvent is added to a good solvent containing a crude compound or the good solvent containing a crude compound is added dropwise to the poor solvent to precipitate the target substance. This method, therefore, has a low purification ability in principle, causing difficulty in removing the impurities described above. In addition, it is difficult to appropriately control the volume ratio between the good and poor solvents in industrial production. An inappropriate ratio generates an oily or viscous precipitate. As a result, purification is often unsuccessful. In fact, according to a method disclosed in Japanese Unexamined Patent Application Publication No. 60-152495, the reprecipitation process produces the target substance in the form of a viscous liquid, which is unsuitable for industrial applications. Methods for amorphization by reprecipitation have been disclosed, but no method for generating a crystal by crystallization is known.